Hydrocarbon distillates and residual-containing oils having characteristics which render them otherwise suitable for use as fuels for compression ignition or diesel engines, or other atomizing or vaporizing type burners, frequently have ignition characteristics that render them unsuitable or only poorly suitable for such use. Fuels that have poor ignition characteristics, that is, relatively high spontaneous ignition temperatures, will exhibit an unduly large ignition lag between the time the fuel is injected into a zone of combustion and the time when the fuel ignites. In diesel engines, for example, a large ignition lag will result in combustion of the fuel and the development of pressure over an improper portion of the crank angle period and piston stroke, resulting in knocking, rough engine operation, incomplete combustion in the combustion zone, power loss, and ultimately detriment to the engine.
To overcome these ignition or combustion problems, the fuel may be refined to produce a higher proportion of straight chain hydrocarbons similar to the original industry standard, cetane. This is, however, costly and various treating agents, such as octyl nitrate, have been used to improve ignition and combustion characteristics. The ignition quality of a diesel fuel is normally expressed in terms of its cetane number. The cetane number of a given fuel is defined as the percent proportion of cetane (a fast burning C.sub.16 paraffinic constituent) in alphamethyl-napthtalene (a slow burning aromatic material) that will match the performance of the fuel at the same compression ratio in a standard test engine.